Heat shrinkable cylindrically laminated films comprising 3-5 layers, one of which is a polyvinylidene chloride type resin (hereinafter referred to as PVDC) as an oxygen gas barrier layer and having a total thickness of 40-80 are well known in the art. Typical laminated films which are currently commercially available include, for example, a laminated cylindrical film comprising 3 layers of an ethylene-vinyl acetate copolymer resin (hereinafter referred to as EVA) at the exterior side of the cylinder/a PVDC/a cross-linked EVA at the interior side of the cylinder according to Japanese Patent Application Publication No. 43024/83 (U.S. Pat. No. 3,741,253); a laminated cylindrical film comprising 3-5 layers of a specific polyolefin resin blend at the exterior side of the cylinder/a PVDC/a specific polyolefin resin blend at the interior side of the cylinder according to Japanese Patent Application Publication No. 40988/85 (U.S. Pat. No. 4,390,587); and a laminated cylindrical film comprising 4-5 layers of an EVA/a PVDC/an ionomer resin at the interior side of the cylinder according to Japanese Patent Application Publication No. 2192/80 (U.S. Pat. No. 4,161 ,562). These laminated cylindrical films have occupied the market share as the packaging materials used for packaging in tight irregularly shaped fatty foods as contents by heat shrinking by proposing as the sales point their unique advantages such as excellent appearance owing to their transparency, their heat shrinkability at low temperature which will not damage the freshness of contents, excellent handling and working properties and the like.
However, with the proceeding of the improvement of packaging efficiency by the introduction of high speed packaging technique using a rotary chamber vacuum packaging machine, new problems of quality have happened in the packaging field, which problems are those of quality of design and cannot be solved with the aforementioned packaging materials.
In other words, as the result of research of the present inventors, the applicability of the high speed packaging technique may be decided by the following three points; first of all, possibility of making a certain sealed part having oil resistance while shortening the time required for tight sealing of packing bags filled with contents under reduced pressure; secondly, securing the cruel treatment resistance of the films so as to resisting rough handling of packaged articles accompanied with the increase of packaging speed under severe conditions; and third, enhancement of oil resistance of interior and exterior layers, in particular, on the treatment at high temperature. In addition, these new quality requirements must be satisfied without damaging the conventional quality level and is also in a high level.
Specifically, referring to the cruel treatment resistance as an index of causing no failures such as pin hole or break of bag on handling of a package, the requirement level has been raised and a commercially available film comprising EVA/PVDC/crosslinked EVA which has been evaluated to have a toughness to ensure that the film can be used for direct packaging of meat with sharp bone at the initial application (see the effect of Japanese Patent Application Publication No. 43024/83) is now evaluated in the new market requirements to be insufficient in toughness owing to high incidence of failures such as pin hole or break of a sealed part during handling the film even if the content is changed to meat without bone so that sharp extrusion will not be detected in it on touching with hand.
However, these films are not easily modified or improved. The reason depends on that a PVDC layer as a core layer will be yellowed or deteriorated upon irradiation of electron rays and thus it is difficult to carry out crosslinking in a state of a laminate, which makes it impossible to employ coextrusion technique excellent in interlaminar strength. Furthermore, it is very difficult to form a cylindrical coextrusion laminate using the PVDC layer and finally to make a laminated film the exterior surface side of which is crosslinked. There has also been proposed a non-crosslinked resin obtained by the examination of resin species to be used and its laminate construction, but the counterproposal is inferior in effect to that of using a crosslinked layer which improves both film forming property by drawing and film property itself. Accordingly, a laminated film containing as a modifying layer a polyolefin type resin layer having a sufficient degree of crosslinking must be prepared by the method such as described in FIG. 1 of U.S. Pat. No. 3,741,253, in which a cylindrical EVA layer is extruded and crosslinked with radiation, a PVDC layer and an EVA layer are cylindrically extruded sequentially on the EVA layer, and these layers are melt-extruded from a coating die in the outside of the die to form a directly adhered laminate with each other, which is drawn by an inflation method to form a cylindrical laminated film. This method has disadvantages in that it is not economical as it requires a technique of a high order and that the crosslinked layer of the film obtained will be an innermost layer of the cylindrical film.
As disclosed in Japanese Patent Application Kokai (Laid-Open) No. 23752/87 (by virtue of a right of priority based on U.S. patent application No. 735,082/85), the method of irradiating electron rays over the whole layers of a laminated film which has been prepared by coextrusion and drawing may be appreciated in the point that an effect of crosslinking-drawing is abandoned and both the modificating effect of crosslinking on the polyolefin type resin and the improving effect on the bonding between the polyolefin type resin and the PVDC layer are utilized. But the method has disadvantages that the PVDC layer is inevitably deteriorated by the irradiation of electron rays (lowering of oxygen gas barrier properties) and that the effect of crosslinking-drawing to be abandoned is very large. Further, it has been confirmed that in the laminated film obtained by irradiating the whole layers typified by the films disclosed in Japanese Patent Application Kokai (Laid-Open) No. 143086/76 (U.S. Pat. No. 4,044,187) and U.S. Pat. No. 3,821,182 the PVDC layer is deteriorated drastically (see Tables 4 and 5).